But you're not going to beat someone up, are you? :)

These are all correct! 1 and 4 kind of implies that someone else will be beating the person up but, it can also mean that you are beating them if its clear from context and the word that's stressed. 2 sounds more dramatic, like a line people say in movies. 3 is the most natural sounding but but personally, as a native speaker I'd tweak 3 like this to sound even more natural: "I'm gonna beat up whoever did this."

Hello Mohamed, They all sound natural, but they have different meanings or connotations. 1 & 4 are identical in meaning and connotation, mostly. They mean that someone will suffer injury because of what they did, but the person inflicting that injury is not identified. 4 - Sometimes when someone says something like this, they mean that they are angry and they're the one who will do the beating, but 1 is not used this way. 2 & 3 are identical in meaning and connotation. They mean that someone will suffer injury because of what they did, and you are the person who will inflict that injury. 2 is also a more dramatic way to say it than 3, in my opinion. 2 needs a comma after the word THIS, for correct punctuation.